There are many benefits to including superconductive elements in electronic circuitry. Superconductivity refers to that state of metals and alloys in which the electrical resistivity is zero when the specimen is cooled to a sufficiently low temperature. The temperature at which a specimen undergoes a transition from a state of normal electrical resistivity to a state of superconductivity is known as the critical temperature (T.sub.c). The use of superconductive material in circuits is advantageous because of the elimination of normal electrical resistive losses.
In the past, attaining the T.sub.c of known superconducting materials required the use of liquid helium and expensive cooling equipment. In 1986 a superconducting Bednorz and Muller, Possible High Tc Superconductivity in the Ba--La--Cu--O System, 64 Z.Phys. B-Condensed Matter 189 (1986). Since that announcement superconducting materials having higher critical temperatures have been discovered. Collectively these are referred to as high temperature superconductors (HTSs). Currently, HTSs having critical temperatures in excess of the boiling point of liquid nitrogen, 77 K (i.e. about -196.degree. C. or about -321.degree. F.) at atmospheric pressure, have been disclosed.
Superconducting compounds consisting of combinations of alkaline earth metals and rare earth metals such as barium and yttrium in conjunction with copper (known as "YBCO superconductors") were found to exhibit superconductivity at temperatures above 77 K. See, e.g., Wu, et al., Superconductivity at 93 K in a New Mixed-Phase Y--Ba--Cu--O Compound System at Ambient Pressure, 58 Phys. Rev. Lett. 908 (1987). In addition, high temperature superconducting compounds containing bismuth have been disclosed. See, e.g., Maeda, A New High-Tc Oxide Superconductor Without a Rare Earth Element, 37 J. App. Phys. L209 (1988); and Chu, et al., Superconductivity up to 114 K in the Bi--Al--Ca--Br--Cu--O Compound System Without Rare Earth Elements, 60 Phys. Rev. Lett. 941 (1988). Furthermore, superconducting compounds containing thallium have been discovered to have critical temperatures ranging from 90 K to 123 K (some of the highest critical temperatures to date). See, e.g., Koren, et al., 54 Appl. Phys. Lett. 1920 (1989).
These HTSs have been prepared in a number of forms. The earliest forms were preparation of bulk materials, which were sufficient to determine the existence of the superconducting state and phases. More recently, HTS thin films on various substrates have been prepared which have proved to be useful for making practical superconducting devices.
It is desirable to use HTS films for microwave and RF applications. Optimally, such HTS films must have low surface resistance and must be able to handle significant power levels. It is preferred that HTS films used for microwave applications have a high T.sub.c and have a substantially linear response at high power levels (i.e. R.sub.s does not significantly vary with RF current density). Many of these criteria are satisfied by HTS films which have both a low fault density and are epitaxial. Epitaxy refers to that state of a film wherein there is a systematic and single (or uniform) orientation of the crystal lattice of the film with respect to the substrate. However, not many films exhibit linear performance characteristics.
Epitaxy may even be necessary for certain HTS films to exhibit desirable microwave and RF applications properties. For example, epitaxial TBCCO thin films (i.e. films containing thallium, barium, optionally calcium, and copper oxide) exhibit desirable microwave properties. See Eddy, et al., "Surface Resistance Studies of Laser Deposited Superconducting Tl.sub.2 Ba.sub.2 Ca.sub.1 Cu.sub.2 O.sub.8 Films," 70 J. Appl. Phys. 496 (1991). Examples of various phases of such TBCCO thin films which are known to exist include Tl.sub.2 Ca.sub.1 Ba.sub.2 Cu.sub.2 O.sub.8 (i.e. 2122), Tl.sub.2 Ca.sub.2 Ba.sub.2 Cu.sub.3 O.sub.10 (i.e. 2223), Tl.sub.1 Ca.sub.1 Ba.sub.2 Cu.sub.2 O.sub.7 (i.e. 1122), and Tl.sub.1 Ca.sub.2 Ba.sub.2 Cu.sub.3 O.sub.9 (i.e. 1223). See Olson, et al., "Preparation of Superconducting Tl--Ca--Ba--Cu Thin Films by Chemical Deposition," 55(2) Appl. Phys. Lett. 188 (1989); and Beyers, et al., "Crystallography and Microstructure of Tl--Ca--Ba--Cu--O Superconducting Oxides," 53(5) Appl. Phys. Lett. 432 (1988). However, TBCCO thin films having optimal or uniform nucleation grow on only a few substrates (e.g. such TBCCO thin films have been shown to grow on LaAlO.sub.3 and SrTiO.sub.3).
In addition, LaAlO.sub.3 is the only substrate at present upon which TBCCO films suitable for microwave applications may be grown, but such films exhibit non-linear performance characteristics. Also disadvantageously, LaAlO.sub.3 substrate exhibits a high dielectric constant which is variable due to twinning. These properties of LaAlO.sub.3 make microwave design difficult. For example, microwave design and microwave performance modelling is limited by the use of LaAlO.sub.3 as a substrate. The high dielectric constant results in line widths at high frequencies which are narrow, and excessive twinning limits frequency setability to .+-.1 percent.
Important microwave and RF applications require twin-free, low dielectric constant substrates. In addition, it is preferred that the substrates be available in large areas (two inch round or greater) to fabricate devices such as narrow band filters at desired frequencies. Substrates such as MgO satisfy these criteria. However, good HTS thin films which are thallium and copper oxide based, such as TBCCO, do not easily grow epitaxially on MgO. Often, such HTS films grow on MgO in several orientations, resulting in high angle grain boundaries and degraded superconducting properties.
The prior art has failed to provide a combination of both a good HTS film and a good substrate which exhibit properties suitable for microwave and/or RF applications. Specifically, the prior art does not provide a device suitable for such applications comprising a HTS film having low surface resistance, exhibiting substantially linear response characteristics at high RF current density (i.e. R.sub.s does not significantly vary with RF current density), and which can be grown on a substrate which has a low dielectric constant, which is twin-free, and which does not degrade the superconducting properties of the film.